Growth in animals is believed to be regulated by a cascade of bio-regulatory molecules. The hypothalamus produces a substance called Growth Hormone Releasing Factor (GRF) which in turn acts upon the pituitary to cause release of growth hormone (GH). GH stimulates the secretion of insulin growth factor (IGF) from the liver and other peripheral organs which binds to various cellular receptors stimulating the events required for linear growth. The pituitary is maintained under negative feedback control by somatostatin and IGF. GRF has been found to be enormously active and capable of stimulating the release of microgram per ml. levels of growth hormone in the blood. GRF can be utilized therapeutically in most of the areas now considered candidates for treatment by growth hormone, for example treatment of pituitary dwarfism, diabetes resulting from growth hormone production, enhancement of wound healing, treatment of burns or retardation of aging process.
The successful isolation of GRF was due partly to the discovery that pancreatic tumors associated with acromegaly ectopically produced large quantities of GRF. Three forms of GRF, consisting of peptides homologous from the amino-terminus of 44,40 and 37 amino acids, were isolated by Guillemin et al [Science 218, 585-587 (1982)] and Rivier et al [Nature, 300, 276-278 (1982)]. The 44 amino acid amidated form of GRF, is considered to be the parent molecule and exhibits the full intrinsic activity and highest potency of the aforesaid forms of this molecule. The amidated carboxy-terminus is a key structural requirement for this high level of activity as the corresponding free acid (GRF(1-40)-OH) has a substantially lower level of activity. This is an important factor in developing low cost processes to produce these clinically important molecules.
Thus, since amidation of recombinant DNA produced peptides have not previously been possible by methods which could be conveniently employed in high yield steps, the preparation of the desired product, GRF(1-44)-NH.sub.2, could previously be made only by use of conventional solid phase or solution phase peptide synthesis methods. The preparation of such a large peptide by these methods still represents a formidable technical challenge and the cost of production remains relatively high.
It is well recognized in the art that peptides can be produced in large scale and at lowest cost by employing recombinant DNA technology. Thus, it would be an important development in the commercialization of GRF(1-44)-NH.sub.2 to be able to use a recombinantly produced peptide as substrate for the introduction of the amide functionality.